Shafts intended as torque-carrying members have been formed of metal matrix composite (MMC) materials. One example is a titanium composite shaft containing reinforcing fibers, such as silicon carbide or boron fibers. The shaft is fabricated by forming composite sheets of titanium foil and fibers using known consolidation processes, wrapping the resulting composite sheets around a mandrel, and then consolidating the sheets at high temperatures and pressures. While acceptable in many respects, this fabrication process does not easily lend itself to high volume production or allow uniform fiber distribution necessary to obtain the full benefit of the composite structure. Nor does the process enable the achievement of precise and repeatable material distribution and runouts required to balance the shaft. Accurate and repeatable balancing is an absolute requirement of particular concern for turbine output shafts of gas turbine engines. Finally, titanium matrix materials are generally not suitable for mechanical coupling features such as shaft splines, which necessitates separate fabrication steps during which an attachment with an appropriate coupling feature is brazed to one or both ends of the shaft. The separate fabrication steps are undesirable from the standpoint of production yields and processing costs.
Given the state of the art, it should be noted that composite materials offer a potential avenue to increase power output shaft stiffness, which is critical in terms of the dynamic performance of gas turbine engines. For example, higher shaft stiffness permits higher operational (turbine) speeds before critical speeds are reached, which allows for fewer turbomachinery stages and better turbine performance over larger bearing support spans. As a result, the stiffness of a turbine output shaft of a small gas turbine engine affects in one way or another the design of the compressor (e.g., axial or centrifugal), the number of turbine stages, type of rotor support bearings, bumper bearings on critical shafting, and bearing cooling and clearance control. A very high-stiffness output shaft can make possible significantly improved and lower-cost engine designs. However, shafts that exhibit greater stiffness and acceptable dynamic balance have been difficult to achieve.